linux-kernel - Re: [PATCH V2 4/6] ARM: Make PID_IN_CONTEXTIDR incompatible with PID

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Message-ID: <52E2A866.1050400@codeaurora.org>
Date:	Fri, 24 Jan 2014 12:52:38 -0500
From:	Christopher Covington <cov@...eaurora.org>
To:	Will Deacon <will.deacon@....com>
CC:	Adrien Vergé <adrienverge@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Mathieu Poirier <mathieu.poirier@...aro.org>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 4/6] ARM: Make PID_IN_CONTEXTIDR incompatible with
 PID_NS

On 01/24/2014 12:17 PM, Will Deacon wrote:
> On Fri, Jan 24, 2014 at 05:16:28PM +0000, Adrien Vergé wrote:
>> 2014/1/24 Will Deacon <will.deacon@....com>:
>>> Are you sure about this? The value we write is actually task_pid_nr, which I
>>> believe to be globally unique.
>>
>> You are right: the task_pid_nr is unique in the system. However when
>> using namespaces, the so called "PID" is the virtual number that
>> processes in different namespaces can share.
>>
>> This PID is the one visible by user-space tasks, in particular
>> user-space tracers and debuggers. These programs would expect to find
>> the PID of the traced process in the Context ID reg, while it is not.
>> I think it is better to remove confusion by making PID_IN_CONTEXTIDR
>> and PID_NS incompatible.
>>
>> What do you think?
> 
> I think I'd rather have the global ID than disable a potentially useful
> feature, especially since this is likely to be consumed by external trace
> tools as opposed to user-space tasks.

We've discussed before that the ARM architecture doesn't say what should be
written to the CONTEXTIDR, so it's up to us to decide. Will has a use case
where the global PID is useful. Adrien's patches present a use case where I
think the virtual PID would be useful. I've done work in the past where
writing the process group ID was useful. Would it be reasonable to make what's
written to the CONTEXTIDR run-time configurable? If so, what would be the best
interface for configuring it?

Thanks,
Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
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